What is the difference between Motor 6D's and Welds?

Motor6D's allow 2 connected parts to move.. think of your character, your character uses motor6D's, if motor6D's were like welds they animations wouldn't be possible, because all the limbs would move relative to the movement of other parts, specifically Part1 would move if Part0 moved..

but with motor6D's, one part can move and rotate.. that's why many limbs of your character can move in different directions..